1. Field of the Invention
The present invention relates generally to an engine exhaust gas purification apparatus. More particularly, the invention is concerned with an apparatus for introducing fresh air into an exhaust gas pipe of an internal combustion engine while heating sufficiently the air introduced into the exhaust pipe, for the purpose of promoting or accelerating the activity of a catalyst employed for purifying an exhaust gas discharged from the engine.
2. Description of the Related Art
For a better understanding of the present invention, the background of the technique to which the present invention belongs will first be described in some detail. FIG. 11 is a view showing schematically a configuration of an internal combustion engine system which is equipped with an apparatus for introducing fresh air into an exhaust pipe. Referring to the figure, an internal combustion engine (hereinafter also referred to simply as the engine) 1 is provided with an air intake pipe 2 for supplying air into the engine 1, the intake pipe 2 having an inlet port connected to an air cleaner 3 which serves for screening out the dusts or other foreign materials from the air introduced into the intake pipe 2. Mounted within the intake pipe 2 is a throttle valve 4 for adjusting or regulating the flow or volume of the air charged into the engine 1 by adjusting the degree of opening of that valve.
An exhaust pipe 5 connected to the engine 1 serves to transport the exhaust gas discharged from the engine 1 to a catalytic converter 6 which is provided for purifying the exhaust gas through a catalytic chemical reaction. The intake pipe 2 is connected to the exhaust pipe 5 at a location upstream of the catalytic converter 6 by way of a shunt pipe 8 for introducing the fresh air into the exhaust pipe 5 from the intake pipe 2. In this conjunction, an air pump 7 is installed in the fresh air introducing shunt pipe 8 for forcibly feeding the air having passed through the air cleaner 3 into the shunt pipe 8 under a pressure. Disposed in the fresh air introducing pipe 8 at a position downstream of the air pump 7 is a check valve 9 which functions to prevent the exhaust gas from flowing backwardly from the exhaust pipe 5 into the intake pipe 2. A transmission 10 is coupled to an output shaft (not shown) of the engine.
In operation of the engine system of the structure described above, clean air passed through the air cleaner 3 is fed through the intake pipe 2 in an amount or flow adjusted in dependence on the degree of opening of the throttle valve 4 to be charged into the engine together with a fuel jetted from a fuel injector (not shown). An air-fuel mixture thus charged undergoes combustion within the engine cylinder, wherein the combustion energy is converted into kinetic energy for driving the output shaft of the engine. The exhaust gas produced as a result of the combustion is discharged into the exhaust gas pipe 5.
On the other hand, the air pump 7 is driven by a power derived from the output of the engine to compress the air tapped from the intake pipe 2, wherein the air discharged from the air pump 7 is fed to the exhaust pipe 5 and hence to the catalytic converter 6 by way of the fresh air introducing pipe 8 and the check valve 9. Within the catalytic converter 6, the fresh air reacts with noxious components contained in the engine exhaust gas such as hydrocarbon (HC in abbreviation) and carbon monoxide (CO) to thereby make the exhaust gas harmless (detoxication). More specifically, a gas mixture of the fresh air and the exhaust gas fed to the catalytic converter 6 contains noxious components originating in the exhaust gas such as hydrocarbon, monoxide and the like as well as oxygen and others contained in the air introduced through the fresh air introducing pipe 8. The catalytic converter 6 serves to convert above-mentioned noxious components to water (H.sub.2 O), carbon dioxide (CO.sub.2) and other through catalytic chemical reaction, to thereby purify or detoxify the exhaust gas discharged from the engine.
Parenthetically, the amount or flow of the fresh air introduced into the exhaust pipe 5 through the fresh air introducing pipe 8 is maintained substantially constant at a predetermined value, as can be seen from FIG. 12.
The hitherto known apparatus for introducing the fresh air into the exhaust gas pipe of the engine suffers, however from several problems. First, the fresh air introduced into the exhaust pipe via the fresh air introducing shunt pipe is at a relatively low temperature on the order of the normal or ambient temperature. Consequently, activity of oxygen molecules contained in the fresh air as added remains low, which means that the efficiency of reaction of oxygen with the noxious components of the exhaust gas in the catalytic converter is poor, incurring undesirable degradation in the exhaust gas purification efficiency.
Another problem of the fresh air introducing apparatus known heretofore is seen in that the exhaust gas purification efficiency is necessarily limited by the amount of the fresh air as introduced. In this conjunction, the fresh air introduced into the exhaust pipe is maintained substantially constant, as mentioned above. Accordingly, it can not be expected to improve or enhance the exhaust gas purification efficiency over the limit imposed with regard to the amount of the fresh air as added.